Synopsis

Altered aortic anatomy after prosthesis
implantation has been shown to increase secondary flow patterns with potential long-term
effects. Therefore, patients after valve-sparing aortic root and ascending
aorta replacement with a physiologically pre-shaped prosthesis were examined
with 4D-Flow-MRI and compared to patients with straight grafts and age-matched
volunteers. A reduced angulation at the distal anastomosis as well as a slightly
reduced intensity of secondary flow patterns was confirmed. However, there was
no reduction of secondary flow patterns in comparison to straight prostheses
potentially attributed to a residual angulation at the proximal anastomosis and
a dilatation at the distal anastomosis.

Background and Purpose

The David-procedure is a standard valve-sparing
technique for surgical repair of aortic root aneurysms (VSARR). If the aneurysm
extends into the ascending aorta, an additional prosthesis of the ascending
aorta (AAoR) is implanted. Traditionally, a straight graft is used to replace
the physiologically curved ascending aorta. As a result, a kinking at the proximal
or distal anastomosis of the AAoR can frequently be observed1. The association between altered
aortic arch anatomy and an increased number of secondary flow patterns was
shown in previous 4D-Flow-MRI studies2-5. This holds the potential risk
of thromboembolisms and unwanted vessel wall remodeling. Physiologically pre-shaped,
90° bent AAoR-prostheses promise to reduce kinking and associated flow
alterations. Therefore, it was the purpose of this work to compare flow
patterns in patients with VSARR and 90° curved AAoR prostheses (CP; Uni-Graft W
Aortic Arch, Braun, Germany) to those with VSARR and straight prostheses (SP) and
age-matched, healthy volunteers (VOL).

Data
analysis: GTFlow (v2.1.15, GyroTools, CH) was used to render the vessel wall and to
visualize blood flow using streamlines and time-resolved particle traces, both
color-coded with respect to the acquired velocities. Offline data processing
included aliasing correction in 5 datasets using PhaseUnwrappingTool (v1,
Fraunhofer MEVIS, GER). Aortic geometries known to influence hemodynamics (round,
gothic, cubic6,7), presence of kinking, and
diameters were assessed. Secondary flow patterns8 (vortices, secondary helices) in
the thoracic aorta were recorded and graded as 1: <33%, grade 2: ≥33% and
≤66%, and grade 3: >66% according to each pattern’s extent in relation to
the vessel diameter. Due to the sample size, statistical significance was tested
applying the Mann-Whitney-U-test.

Results

All studies and flow pattern
analyses were successfully accomplished. As seen in Fig. 1, angular forms of
the thoracic aorta predominated in patients: 67% (CP) and 75% (SP) presented
with a cubic form, 22% (CP) and 25% (SP) revealed a gothic aortic arch. Only
one CP-patient had a round aortic arch. In contrast, round forms dominated in
volunteers (83%). In CP-patients there was significant reduction of a kinking
at the distal anastomosis in comparison to patients with straight grafts: 22%
(CP), 75% (SP) (p=0.04), see also Fig. 2. All patients but the aforementioned CP-patient
with a round arch showed an angulation between the prostheses of aortic root and
ascending aorta. At the distal anastomosis, between straight or pre-shaped
graft to the aortic arch, there was a dilatation of 0.5±0.4cm (CP) and 0.6±0.4cm
(SP) (n.s.), see also Fig. 3.
Patients developed more secondary
flow patterns than volunteers (CP: n=3.1±1.5; SP: n=3.4±1.1; VOL: n=1.4±0.8;
p=0.002). The most pronounced increase of secondary flow patterns in patients
was in the region of the prosthesis and directly distal to it (CP: n=2.2±1.4;
SP: n=2.3±1.0; VOL: n=0.3±0.5; p<0.001), Fig. 2. There was no significant
difference between prosthesis types (p=0.728). The only CP-patient with a round
aortic arch developed no prosthesis-related flow patterns but a small,
physiological vortex in the ductus diverticulum. In CP secondary flow patterns
in and directly distal to the prosthesis tended to be less pronounced than in SP
(median [25%,75%], CP: 2[2,2], SP: 2[2,3]; p=0.088).

Discussion and Conclusion

The anatomically pre-shaped 90° prosthesis resulted
in a reduced kinking at the distal anastomosis and slightly reduced intensity,
yet no decrease in number of secondary flow patterns in comparison to patients
with a straight graft. The constant number of secondary flow patterns despite
an improved postoperative geometry can be explained by the residual angulation
at the proximal anastomosis of the curved prosthesis and a post-prosthetic
dilatation. This is underlined by findings in a single patient with a curved
prosthesis who revealed both a near-physiological geometry and flow patterns
comparable to age-matched volunteers.

Acknowledgements

The authors express their gratitude toward Mrs. Martina Schroeder for her skillful assistance and Dr Gerard Crelier for his continuous support.

Figures

Figure 1
depicts the geometic variations in aortic arch geometry that was categorized as
either round, cubic, or gothic. In patients, angular forms (cubic, gothic)
predominated. In volunteers, round forms were most frequent. Only one patient
with a curved graft had a round aortic arch, comparable to volunteers (see also
Fig. 2B).

Figure 2. Representative flow pattern analysis using pathlines in a VSARR/curved
AAoR-patient (CP) with a kinking at the proximal anastomosis (A) and a patient
with VSARR/straight prosthesis (C) depicting kinkig at the proximal and distal anastomosis (arrows)
and associated secondary flow patterns (dashed lines). In (A), an additional
vortex in a post-prosthetic dilatation is observed. For reference, a normal,
undisturbed flow pattern of an age-matched volunteer (D) and the single CP-patient
with a round aortic arch without prosthesis-related secondary flow patterns (B)
can be appreciated. Prosthesis locations are indicated with dotted lines.

Figure 3 depicts the diameter change between the prosthesis lumen and the
native aorta at the distal anastomosis. Red lines depict the mean value of diameter
change, the green line indicates the single patient with a curved prosthesis
with near-physilogical geomatry (see als Fig. 2B).